High strength Mg based alloy and Mg based casting alloy and article made of the alloy

ABSTRACT

A high strength Mg based alloy and a Mg based casting alloy having a good fluidity and a good mechanical property, and are used to provide a molded article using the alloy. A high strength Mg based alloy, which contains 12 to 20% of Al by weight; 0.1 to 10% of Zn; 0.1 to 15% of Sn; and 0.05 to 1.5% of Mn is used by being injection molded.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation application of U.S. Ser. No.09/727,535, filed Dec. 4, 2000, the contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a novel Mg based alloy and anovel Mg casting alloy capable of mass producing OA parts, car parts,electric appliance parts and so on through die casting, injectionmolding or the like, and relates to articles mold-cast using the alloy.

[0004] 2. Description of the Prior Art

[0005] The casting Mg alloys practically used in present time are asfollows:

[0006] (1) AZ, AM alloys (Mg—Al—(Zn)—Mn system, for example, ASTM:AZ91D);

[0007] (2) AS alloy (Mg—Al—Si—Mn system, for example, ASTM: AS41); and

[0008] (3) AE, QE, WE alloys (an alloy group containing one or morekinds of REM, Ag, Y).

[0009] The alloy (1) is most commonly used as the die casting and theinjection molding Mg alloy, and particularly the AZ91D is good indie-castability and in corrosion resistance and widely applied to carparts and electric appliance parts. The alloys (2), (3) are alloysimproving the mechanical properties such as the creep property and thehigh temperature strength. As the prior art in regard to these alloys,various kinds of alloys are disclosed in the following patent gazettes.

[0010] For example, Japanese Patent Application Laid-Open No. 6-330216discloses an Mg based alloy containing Ca, Si, Al, Zn and Mn, JapanesePatent Application Laid-Open No. 9-104942 discloses an Mg based alloycontaining 5 to 10 of Al, 0.2 to 1 of Si and 0.05 to 0.5 of Cu, andJapanese Patent Application Laid-Open No. 10-147830 discloses an Mgbased alloy containing 1 to 6 of Gd and 6 to 12 of Y.

[0011] With growing needs of thin thickness and high precision of partsin order to reduce in weight and size of potable devices, high fluidityalloys have been required. The alloy (1) of AZ91D described above iscomparatively high in the fluidity, but the molding yield in injectionmolding is not always sufficiently high.

[0012] The alloy groups (2), (3) are prior to AZ91D in the mechanicalproperties such as creep property, strength at high temperature.However, because of the bad fluidity, the alloy groups (2), (3) are aptto cause casting cracks in the molding method of high speed cooling suchas the injection molding method and are bad in castability.

[0013] The fluidity may be improved by raising the temperature of moltenalloy. However, raising of the molten alloy temperature has problems inoxidation of the molten alloy and in shortening of durable lifetime ofthe production machines. Therefore, it is necessary to improve thefluidity by the other method.

[0014] It is known that the solidification structure of AZ91D becomesdendritic when it is cooled in a comparatively slow speed such as atingot casting. As described above, the alloy is designed by placingspecial emphasis on the molten fluidity, and in regard to the propertiesafter solidification, the alloy is designed so that the various kinds ofproperties such as the mechanical properties are optimized on thepremise that the structure of AZ91D becomes dendritic.

[0015] However, in the cases of die casting and injection molding towhich the alloy is widely applied, it is known that the structure aftersolidification becomes the cellular structure not the dendriticstructure because the cooling rate is very fast. Therefore, it isrequired to change the designing method of the conventional alloyingcomposition.

SUMMARY OF THE INVENTION

[0016] An object of the present invention is to provide a high strengthMg based alloy and a Mg based casting alloy having a good fluidity and agood mechanical property, and a cast article using the alloy.

[0017] As a result of various kinds of studies in order to solve theproblems described above, it is found that the melting point of thealloy is lowered and the fluidity is improved by adding appropriateamounts of Al, Sn and Zn to a magnesium alloy, and the present inventionis established.

[0018] The present invention is characterized by a high strength Mgbased alloy, which contains 2 to 20% of Al by weight; 0.1 to 10% of Zn;0.1 to 15% of Sn; and 0.05 to 1.5% of Mn, or preferably the remainderwhich is consisting essentially of Mg.

[0019] The present invention is characterized by a high strength Mgbased alloy, which contains 2 to 20% of Al by weight; 0.1 to 10% of Zn;0.1 to 15% of Sn; and 0.05 to 1.5% of Mn, and has crystal size of 10 to300 μm, or preferably the remainder which is consisting essentially ofMg.

[0020] The present invention is characterized by a high strength Mgbased alloy, which contains 8 to 20% of Al by weight; 0.1 to 5% of Zn;0.1 to 10% of Sn; and less than 1.5% of Mn, and has a tensile strength(x) at 20° C. larger than 240 MPa; and an elongation ratio (y) largerthan 0.5% and at the same time larger than a value calculated byy=−0.295x+78, or preferably the remainder which is consistingessentially of Mg.

[0021] The present invention is characterized by a high strength Mgbased alloy, which contains 12 to 15% of Al by weight; 0.1 to 5% of Zn;1 to 10% of Sn; 0.1 to 0.5% of Mn, and the remainder contains Mg morethan 75%, or preferably the remainder which is consisting essentially ofMg.

[0022] The present invention is characterized by a high strength Mgbased alloy, which contains 12 to 15% of Al by weight; 0.1 to 5% of Zn;1 to 10% of Sn; 0.1 to 0.5% of Mn; one kind or more than two kinds ofelements selected from the group consisting of Ca, Si and rare-earthelements of which the total content is less than 5%; at least one kindof element selected from the group consisting of Sr and Sb of which thetotal content is less than 1%; or preferably the remainder which isconsisting essentially of Mg.

[0023] The present invention is characterized by a Mg based castingalloy, which contains 2 to 20% of Al by weight; and 0.1 to 15% of Sn; orpreferably the remainder which is consisting essentially of Mg.

[0024] The present invention is characterized by a Mg based castingalloy, which contains 2 to 20% of Al by weight; 0.1 to 10% of Sn; andless than 1.5% of Mn, or preferably the remainder which is consistingessentially of Mg.

[0025] The present invention is characterized by a Mg based castingalloy, which contains 10 to 15% of Al by weight; 0.5 to 3% of Zn; 1.5 to4.5% of Sn; 0.05 to 0.5% of Mn, or the remainder which is consistingessentially of Mg.

[0026] The present invention is characterized by a Mg based castingalloy which is prepared by that the above-mentioned Mg based castingalloys are added with one kind or more than two kinds of elementsselected from the group consisting of Ca, Si and rare-earth elements ofwhich the total content is less than 5% by weight; and at least one kindof element selected from the group consisting of Sr and Sb of which thetotal content is less than 1%, or the remainder which is consistingessentially of Mg.

[0027] The present invention is characterized by a die cast article orinjection molding article, which is casted using a molten metal of anyone of the alloys is described above.

[0028] The present invention is characterized by a thixotropic moldarticle, which is molded using a molten metal of a mixture of liquidphase and solid phase of any one of the alloys described above.

[0029] In detail, it is preferable that the magnesium based alloysdescribed above are formed in desirable shapes through die casting byinjection molding.

[0030] The magnesium alloys in accordance with the present invention areimproved in the fluidity due to lowering of the melting pointparticularly by adding a small amount of Sn to the Mg based alloycontaining Al, and accordingly members having less surface defects canbe obtained. Further, since low temperature molding can be performed andaccordingly the contraction at solidifying is small, members having ahigh dimensional accuracy can be obtained. Therefore, the molding yieldcan be largely improved.

[0031] Further, since the load to the machines, for example, thecylinder of an injection molding machine or the like is decreased, thedurable lifetime of the heat resistant materials can be lengthened.

[0032] Furthermore, the magnesium alloys in accordance with the presentinvention are good in mechanical property and corrosion resistancebecause of the homogeneous and fine microstructure.

[0033] For the purpose of solid-solution hardening, precipitationhardening and improvement of fluidity, the element Al is added above 2%,preferably above 8%, particularly preferable above 12%. However, anexcessive addition exceeding 20% of the element Al produces a largegrain Mg—Al intermetallic compound to substantially decrease theelongation of the molded products. Further, in the casting method havinga high cooling rate such as the die casting or the injection molding,the solidified structure becomes finer as the content of Al isincreased, and the Mg—Al intermetallic compound does not growlarge-sized, but is finely distributed in the crystal grain boundaries.This effect becomes obvious particularly when Sn is added together. Inorder to make the elongation above 3.5% and the tensile strength above265 MPa, it is preferable to add 12 to 17% of Al.

[0034] Further, the element Al in the magnesium alloy in accordance withthe present invention is solved in the α-Mg phase, and reduce themelting point of the alloy. Further, the element Al is solid-solved inthe α-Mg phase and crystallizes the Mg—Al intermetallic compound, withthe result that the strength at room temperature of the alloy isimproved. Furthermore, the element Al suppresses oxidation of the moltenalloy, and improves fluidity of the molten alloy. In order to attainthese effects, the Al content is above 12%, and preferably above 15%.

[0035] The element Sn is solved in the α-Mg phase, and reduce themelting point of the alloy with a small amount of nearly 0.1%,particularly more than 0.5%. Further, the element Sn is solved in theα-Mg phase and crystallizes the Mg—Sn intermetallic compound, as aresult the strength at room temperature is improved. Furthermore, theeffect of Sn on lowering the melting point becomes obvious particularlywhen Al and Zn are added together, but the effect is almost saturatedwhen the Sn content becomes 5%. Further, when the Sn content exceeds15%, the elongation is largely decreased, and the density of the alloybecomes large, and lose the advantage of lightness of the magnesiumalloy. Particularly, the Sn content needs to be lower than 10% in orderto keep the elongation above 3.5%, and the Sn content needs to bepreferably lower than 8% in order to keep the elongation above 4%. Whenthe Sn content is 1 to 7%, it is possible to obtain an alloy having bothof high strength and high elongation.

[0036] The element Zn is added above 0.1% in order to improve thestrength at room temperature and the castability. However, when the Zncontent exceeds 10%, casting cracks are apt to occur. It is preferablethat the Zn content is within a range of 0.1 to 5%, preferably 1 to 5%in which the strength is high and the casting cracks do not occur.

[0037] The element Mn improve the corrosion resistance, this is becauseMn forms a intermetallic compound with Al, and fix Fe in theintermetallic compound, the element Fe being contained in the alloy asan impurity deteriorate the corrosion resistance. When the Mn contentexceeds 1%, the Al-Mn group intermetallic compound excessively depositedand cause an evil effect on the mechanical property, the upper limit ofMn content is set to 1%. Particularly, for the corrosion resistance, Mncontent is effective above 0.05%, and preferably 0.1 to 0.5%.

[0038] The alloy in accordance with the present invention furthercontains at least one element selected from the group consisting of Ca,Si and rare-earth elements, the content of the one kind or in totalbeing less than 5%; and at least one element selected-from the groupconsisting of Sr and Sb, the content of the one kind or in total beingless than 1%. The elements Ca and Si and rare-earth elements areeffective to lower the melting point because these elements formeutectic groups with Mg. However, since addition of these elementsdeteriorates the casting property, the upper limit of the content is 5%.Particularly, it is preferable that the content is above 0.1% and theupper limit is set to 3%.

[0039] The elements Sr and Sb make the metallic structure fine, and toimprove the mechanical properties. The effect of elements Sr and Sb isincreased when the element Si or Ca is added together. The effect ofelements Sr and Sb is increased as the content is increased, but theeffect is saturated when the content exceeds 1%. Therefore, the upperlimit is set to 1%. Particularly, it is preferable that the content isabove 0.03%, and the upper limit is set to 0.5%.

[0040] The Mg based alloy in accordance with the present invention ischaracterized that the surface is covered with an oxide film whichcontains Mg of 15 to 35% by atoms; preferably 20 to 30%, and Mo of 5 to20%. The Mg based alloy in accordance with the present invention ischaracterized that the surface is covered with an oxide-film whichcontains Mg of 15 to 35% by atoms; Mo of 5 to 20; and metallic Al ofless than 30%, preferably 10 to 25%. The Mg based alloy in accordancewith the present invention is characterized that the surface is coveredwith an oxide film which contains Mg of 15 to 35% by atoms; Mo of 5 to20; oxide Al of less than 15%; and metallic Al of less than 15%,preferably 4 to 12%. The Mg based alloy in accordance with the presentinvention is characterized that the surface is covered with an inertoxide film of which a natural immersion electric potential 30 minutesafter immersing into an aqueous solution of 0.01 mol Na₂B₄O₇, pH 9.2,25° C. is higher than −1500 mV, preferably higher than −1400 mV. The Mgbased alloy in accordance with the present invention is characterizedthat the surface is covered with an oxide film of which a naturalimmersion electric potential 15 minutes after immersing into an aqueoussolution of 0.01 mol Na₂SO₄, 25° C. is higher than −1500 mV, preferablyhigher than −1400 mV. Further, the Mg based alloy in accordance with thepresent invention is characterized that the surface is covered with theabove-described oxide film or a specified oxide film, and a waterrepellent organic film containing fluoride is further coated on theoxide film.

BRIEF DESCRIPTION OF DRAWINGS

[0041]FIG. 1 is a cross-sectional view showing the construction of aninjection molding machine used in the present embodiment.

[0042]FIG. 2 is a microscopic photograph showing the metallic structureof a magnesium based alloy ingot fabricated in the embodiment 1.

[0043]FIG. 3 is a graph showing the relationship between Sn content andmelting point.

[0044]FIG. 4 is a diagram that shows the relationship between thecylinder temperature and fluidity lengths for both the magnesium-basedalloy No. 2 in the present invention and an alloy AZ91D in a prior art.

[0045]FIG. 5 is a graph showing the relationship between Sn content andVickers hardness.

[0046]FIG. 6 is a graph showing the relationship between Sn content andtensile strength.

[0047]FIG. 7 is a graph showing the relationship between Sn content andelongation ratio.

[0048]FIG. 8 is a graph showing the relationship between Al content andtensile strength.

[0049]FIG. 9 is a graph showing the relationship between Al content andelongation.

[0050]FIG. 10 is a SEM photograph showing the metallic structure of amagnesium based alloy ingot fabricated in the embodiment 2.

[0051]FIG. 11 is a graph showing the relationship between elongation andtensile strength.

[0052]FIG. 12 is a graph showing the relationship between elongation andtensile strength.

[0053]FIG. 13 is a graph showing the salt spray test result for variousMg based alloys.

[0054]FIG. 14 is a perspective view showing a note-shaped personalcomputer.

[0055]FIG. 15 is a perspective view showing a mobile type liquid crystalprojector.

[0056]FIG. 16 is a perspective view showing a home electric vacuumcleaner.

[0057]FIG. 17 is a perspective view showing an impeller.

[0058]FIG. 18 shows a perspective view of a portable telephone apparatusto which a magnesium-based alloy disclosed in the embodiment 1 in thepresent invention is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

[0059] A magnesium chloride type flux was applied on the inner surfaceof a melting pot made of casting iron pre-heated in an electric furnace,and raw materials were put into the melting pot so as to form an alloyhaving a composition (weight %) shown in Table 1 to be melted. Afterstirring the molten metal at 750° C. and removing slag, the molten metalwas cast in a metal mold of 50 mm×50 mm×300 mm pre-heated to 150° C. tofabricate a Mg alloy ingot. During melting work, in order to preventingburning the flux was sprinkled on the molten alloy surface, ifnecessary, Mm is a mischmetal (La50 wt %-Ce50 wt % alloy).

[0060]FIG. 2 shows a typical metallic structure of an alloy obtainedthrough the method as described above. Mg—Al compound phase (whiteportions) are crystallized in a network shape in the α-phase grainboundary, and Mg—Sn compound phase (black portions) are crystallizedbetween the network of the Mg—Al compound phase.

[0061]FIG. 3 shows measured results of melting points of the alloys,particularly the relationship between the melting point and Sn contentfor the alloys No. 1 to 3 and 11 to 13. The alloy melting point isdecreased as the Sn content is increased, and the effect of Sn additionon the melting point is saturated when the its content exceeds 10 Wt %.However, it can be understood that the melting point of the alloy No. 12containing Al and Zn contents less than the specified values of thepresent invention is small in the melting point fall from AZ91D alloy(No. 11). Further, as shown in the figure, the melting point is steeplydecreased as the Sn content is increased up to the Sn content of 2%, butmildly decrease where the Sn content is above 2%. Further, by making theSn content above 0.5%, the melting point is lower than that 596 OC ofAZ91D.

Embodiment 2

[0062] A magnesium chloride type flux was applied, on the inner surfaceof a melting pot made of casting iron preheated in an electric furnace,and raw materials were put into the melting pot so as to form an alloyhaving a composition (weight %) shown in Table 1 to be melted. Afterstirring the molten metal at 750° C. and removing slag, the molten metalwas cast in a metal mold of 50 mm×50 mm×300 mm pre-heated to 150° C. tofabricate a Mg alloy ingot. During melting work, in order to preventingburning the flux was sprinkled on the molten alloy surface, ifnecessary. An alloy chip of 2 mm to 10 mm diameter was fabricated bymilling the ingot obtained through the method as described above, andused as a raw material for injection molding. A machine having a moldclamping force of 75 t was used for the injection molding to form aninjection molded piece of 120 mm×50 mm×1 mm thickness. The moldingcondition was as follows. A Mm (mischmetal) indicates an alloycontaining 50 wt % La and 50 wt % Ce.

[0063] Injection speed: 1.6 m/s

[0064] Injection pressure: 800 kg/cm²

[0065] Molten metal temperature: alloy melting point+20° C.

[0066] Mold temperature: 150° C.

[0067] Strength evaluation tests (hardness, tensile strength,elongation) were conducted by obtaining the following test pieces fromthe molded pieces obtained as described above.

[0068] Test piece: 1 mm thickness, 12 mm gage length, 16 mm length and10 mm width of parallel part.

[0069] Tensile test: Using an Instron testing machine, measurement wasperformed under the condition of 0.3/mm strain speed and at 25° C.

[0070] The test pieces No. 1 to 10 and 12, 13 are samples each withinthe composition range of the embodiment of the present invention, andthe test pieces No. 11, 14 and 15 are comparative examples out of thecomposition range of the embodiment of the present invention (the testpiece No. 11 is AZ91D standard alloy). TABLE 1 Alloy No. Al Zn Sn MnOthers Mg 1 12 3 1 0.2 — bal. 2 12 3 5 0.2 — bal. 3 12 3 10 0.2 — bal. 412 5 5 0.2 — bal. 5 15 3 5 0.2 — bal. 6 18 3 5 0.2 — bal. 7 20 3 5 0.2 —bal. 8 12 1 3 0.2 Si: 1, Sr: 0.05 bal. 9 12 1 3 0.2 Ca: 2, Sb: 0.05 bal.10 12 1 3 0.2 Mm: 2 bal. 11 8.9 0.76 — 0.24 — bal. 12 11 0 1 0.2 — bal.13 12 3 11 0.2 — bal. 14 21 3 5 0.2 — bal. 15 12 3 5 — — bal.

[0071]FIG. 1 is a cross-sectional view showing the main portion of theinjection molding machine used in the present embodiment.

[0072] An alloy raw material 1 for injection molding is put into ahopper 2 to be supplied into a cylinder 4. The raw material is kneadedand mixed in the cylinder 4 while being transferred toward a nozzle 6 bya rotating screw 5, and at the same time heated by a cylinder heater 7.The alloy raw material is injection molded under a melted state wherethe heated temperature is higher than the liquid-phase line temperature,or under a semi-melted state where a solid phase having a temperaturelower than the liquid-phase line temperature and a liquid phase aremixed. The melted state or semi-melted state molten metal 10 of the rawalloy material transferred to the front portion of the screw 5 is filledinto a metal mold 9 though the nozzle 6 by moving the screw forwardusing a high speed injection mechanism 8. The pressure in the metal moldis kept until the molten metal is solidified, and after being solidifiedthe metal mold 9 is opened to take out the molded article. Referring tothe figure, the screw 5 has a spiral blade 13 on a cylindrical solidbased body 14, and the alloy raw material 1 is heated up to a hightemperature to be made the melted state or the semi-melted statedepending on the temperature of the heater 7 while being kneaded withthe blade 13 by rotation of the screw 5. The reference character 12 is abackflow preventing ring for the molten metal 10.

[0073] The alloy raw material 1 used in the present embodiment isprepared by melting an alloy of each of the compositions under anon-oxidized atmosphere, and then by cutting the formed alloy into chipssmaller than 10 mm to form grains of the raw material.

[0074]FIG. 4 is a diagram that shows the relationship between theinjection temperature and fluidity lengths for both the magnesium-basedalloy No. 2 in the present invention, an alloy AZ91D in a prior art. Thecylinder temperature is the temperature at which alloys were molded fortheir fluidity length verification. The fluidity length of an alloy isthe length for such sound portion in an injection-molded alloy that hasno surface-defects like crack. The alloy No. 2 is a magnesium-basedalloy containing 12% aluminum, 1% zinc, and 5% tin each in weight ratio.

[0075] The fluidity length was verified using a fluidity lengthverification metallic mold having a width of 10 mm, a thickness of 1 mm,and an overall length of 380 mm, into which each alloy to be verifiedwas injection-molded by an injection molding apparatus shown in FIG. 1keeping the verification metallic mold constantly at 200° C.

[0076] As shown in FIG. 4, it is evident that the alloy in the presentinvention has higher fluidity length performance than alloy AZ91D at anytemperature in our investigation.

[0077] In contrast to that the fluidity lengths of alloys in a prior artreaches saturation of about 300 mm at a temperature 600° C., the alloyNo. 12 in the present invention that includes 3% zinc spread itsfluidity length to about 350 mm at 570° C. and another alloy in thepresent invention that includes 1% zinc also spreads to about 350 mm at580° C.

[0078]FIG. 5 to FIG. 7 are graphs showing the relationships between Sncontent and test results of hardness and tensile strength of theinjection molded article made of each of the alloys shown in Table 1. Asshown in the graphs, by adding Sn by 1%, both of the hardness and thetensile strength become above Hv 110 in hardness and above 269 MPa intensile strength, respectively. On the other hand, the elongation ratiois improved until the Sn content is increased up to 5 wt %, but isdecreased when the Sn content exceeds 5%, and is steeply decreased to avalue before adding Sn when the Sn content exceeds 9%.

[0079]FIG. 8 and FIG. 9 are graphs showing results of tensile test whenthe Al content is varied on the basis of the alloy No. 2(Mg-12A1-3Zn-5Sn). As shown in the graphs, the tensile strength isimproved with increasing the Al content, and the tensile above 279 MPacan be obtained up to 17% of Al content. In regard to the elongationratio, the elongation ratio above 1.9% can be obtained up to 20% of Alcontent. However, when the Al content exceeds 20%, the elongation ratiois extremely decreased to a value smaller than 1%, which is impractical.

[0080] As the contents of Al, Zn, Sn in the magnesium alloy areincreased, (Mg—Al group, Mg—Sn group) crystallized in the α-phase grainboundaries are increased. Increase in the amount of the generally causesto lower the elongation. However, addition of Al, Zn, Sn also has aneffect to fine the α-phase, and accordingly the relative ratio of theα-phase grain boundary volume to the intermetallic compound amount isnot largely changed even if the amount of the intermetallic compound isincreased. Therefore, it can be considered that large decrease ofelongation can be suppressed. However, it is considered that the finingeffect is saturated and the elongation is steeply decreased at valuesnear the Sn and Al contents of 10 wt % and 20 wt %, respectively.

[0081]FIG. 10 shows the photograph of the structure of the injectionmolded article made of the alloy No. 2. The α-phase grains having sizeof nearly 1 to 20 μm, mainly less than 5 μm and the Mg—Al compound phasecrystallized in network shape in the grain boundaries are observed.White small nodules are the Mg—Sn compound phase, and it can beunderstood from this photograph that the solidification structure isrefined, and the Mg—Al and the Mg—Sn compound phase are uniformlydistributed.

[0082] In the case where among the magnesium alloys described above, theembodiments of the alloys No. 1 to 3 in accordance with the presentinvention are injection molded by setting the molten alloy temperatureto the same value (620° C.), surface defects of the molded articles ofthe alloys No. 1 to 3 are substantially decreased compared to those ofthe molded article of AZ91D alloy. The reason is that the differencebetween the molten alloy temperature and the melting point becomeslarger by the amount of decreasing the melting point, and accordinglythe fluidity is improved.

[0083] Further, in the case where molding was performed by setting themolten alloy temperature at injection molding to a temperature lowerthan the melting point of the alloy by 10° C., that is, in the casewhere injection molding was performed under the semi-solid state thatthe solid phase and the liquid phase were mixed, the dimensionalaccuracy of the molded article made of each of the alloys was betterthan that of AZ91D alloy.

[0084]FIG. 11 is a graph showing the relationship between tensilestrength and elongation of the Mg based alloy when the Sn content isvaried to the 12% Al-3% Zn alloy. As shown in the graph, although thestrength and the elongation are increased up to the Sn content of 5%,the strength is increased but the elongation is decreased when the Sncontent exceeds 5%. However, the elongation is as high as 0.5% even atthe Sn content of 11%.

[0085] The straight line in the graph is expressed by the elongation (%)(y) and the tensile strength (MPa) (x), the present embodiment has theelongation higher than the value calculated by y=−0.295x+78. Further, itis preferable that the tensile strength and the elongation are higherthan values calculated by the relationships y=−0.295x+82, 85 or 87.

[0086]FIG. 12 is a graph showing the relationship between tensilestrength and elongation of the Mg based alloy when the Al content isvaried to the 3% Zn-5% Sn alloy. As shown in the graph, it can beunderstood that the tensile strength having a value higher than 275 MPacan be obtained by increasing the Al content to 12%, and the elongationhaving a value higher than 0.5% can be also obtained when the Al contentis less than 20.5%. It is also preferable that in this graph, the valuesare set higher than the values calculated by the above-describedrelationship expressed by the elongation ratio (y) and the tensilestrength (x).

[0087]FIG. 13 is a graph showing the corrosion rate of injection moldedarticles made of the present embodiments of alloys No. 2, 5, 6, 7 andthe comparative alloys No. 11, 15 by a salt water spray test (splaying5% NaCl aqueous solution for 360 hours) at 20° C. From the graph, it canbe understood that all the alloys of the present embodiment have bettercorrosion resistance below corrosion mass loss of 0.1 (mg/cm²·day)compared to that of AZ91D alloy (No. 11). Further, it can be alsounderstood that the corrosion resistance is more improved as the Alcontent is higher. Further, as it is clear from the fact that the alloyNo. 2 added with Mn shows better corrosion resistance compared to thatof the comparative alloy No. 15 not added with Mn, addition of a verysmall amount of Mn substantially increase the corrosion resistance.Further, as shown by the alloys No. 5 and 7, it can be understood thathigh corrosion resistance can be obtained by increasing the Al content.

Embodiment 3

[0088]FIG. 14 is a perspective view showing a notebook-size personalcomputer. In a main body 21, there are arranged a keyboard 22 foroperating input means and a switch board unit 23 containinglight-emitting diodes (LEDs) for indicators and a main switch. Theexterior of the main body 21 is composed of a main body upper case 26and main body bottom case 27. The exterior of a display portion 24 iscomposed of a liquid crystal display (LCD) case 41 and an LCD front 42.In the LCD front 42, a display window is opened SO that the displayportion of the liquid crystal screen 25.

[0089] Among these components, the LCD front 42 was molded using thealloy No. 2 by an injection molding machine of 650 t mold clamping forcein order to make the weight light and to improve the stiffness and theheat dissipation. The injection speed was 3 m/sec, molten alloytemperature was 580° C. and metal mold temperature was 200° C. Thedimension of the molded article was 230 mm×180 mm×4 mm, and averagethickness of 0.7 mm. The molded article obtained through such a waycould be formed in a good dimensional accuracy without surface defectsand with good yield. Similarly, the bottom case was fabricated.

Embodiment 4

[0090]FIG. 15 is a perspective view showing a mobile type liquid crystalprojector.

[0091] A main body is composed of a switch board unit 32 containing LEDsfor indicators and a main switch and a projection lens 33, and theexterior is composed of a main body upper case 31 and a main body bottomcase 34.

[0092] Among these components, the main body upper case 31 was castedusing the alloy No. 2 by a hot chamber die-cast machine of 600 t moldclamping force. The molding condition was injection speed of 2.5 m/sec,molten alloy temperature of 600° C. and metal mold temperature of 200°C. The dimension of the molded article was 248 mm×330 mm×100 mm, andaverage thickness of 1.5 mm. Even though the component was comparativelylarge, a good molded article could be formed without filling defects ina thin wall portion nor occurrence of surface defects.

Embodiment 5

[0093]FIG. 16 is a perspective view showing a home electric vacuumcleaner having an impeller using the Mg based alloy, in accordance withthe present invention.

[0094] Referring to FIG. 16, the reference character 51 is a vacuumcleaner main body which contains a control circuit and an electric drivefan and so on, the reference character 52 is a hose connected to asuction nozzle portion of the vacuum cleaner main body 51, the referencecharacter 53 is a hose grip portion, the reference character 54 is anextension pipe connected to an end (the hose grip portion 53) of thehose, the reference character 55 is a nozzle body connected to theextension pipe 54, the reference character 56 is a switch operatingportion arranged in the hose grip portion 53, the reference character 57is a first infrared light emitting portion arranged in the hose gripportion 53, the reference character 58 is a second infrared lightemitting portion arranged in the hose grip portion 53, and the referencecharacter 59 is an infrared light receiving portion arranged on theupper surface of the vacuum cleaner main body.

[0095]FIG. 17 is an exploded perspective view showing the impeller.

[0096] As a molding method of integrating a front plate 61, a rear plate62 and blades 63 in one piece, an injection molding method was employedin the present embodiment. In this method, a light metal raw materialformed in pellets is used similarly to the injection molding method, andkneaded and melted directly inside an injection molding machine withoutusing any melting furnace or the like, and injected into a metal mold toobtain a molded article. In the present embodiment, the front plate 61,the rear plate 62 and the blades 63 integrated in one piece areindividually formed in one piece using the magnesium based alloy shownin Embodiment 1. Solder material layers are provided over all thesurfaces of the front plate 61 and the rear plate 62, and the blades 63are joined with the solder material. The reference character 64 is asuction port. In the present embodiment, the impeller can be obtained bya mixed molten alloy of liquid phase and solid phase using the injectionmolding machine shown in FIG. 1.

[0097] According to the present embodiment, the impeller can be madelight in weight without filling defects even though the wall thicknessis as thin as 0.7 mm, and the air flow resistance can be reduced.Therefore, the rotating speed of 45000 to 50000 rpm can be attained at 1kW consumed electric power, and the suction power can be attained above550 W.

Embodiment 6

[0098]FIG. 18 shows a perspective view of a portable telephone apparatusto which a magnesium-based alloy disclosed in the embodiment 1 in thepresent invention is applied. As illustrated in FIG. 18, the apparatusis comprised of a cover (4) that includes a number display part (2) anda plurality of keys (3), a retractable antenna (5), and a case (6).

[0099] Among these parts, the cover (4) and the case (6) wereinjection-molded out of the alloy No. 2 for reduce weight, and improvingstiffness, heat dissipation, and electromagnetic shielding propertiesusing an injection molding apparatus having mold clamping force of 75ton. The injection speed was 1 m/s and the temperature of the moltenmetal was 580° C. The dimensions of the molded product were 125 mm by 38mm by 8 mm and average wall thickness was 0.5 mm. This alloy has notcaused any filling-defect and surface-defect with acceptable yield inmolding process even in a thin wall product like this embodiment.

Embodiment 7

[0100] A front cabinet of a 21-inch type television set, a steeringwheel core of a vehicle, a case body of a video-camera, a rid of an MDplayer and a case body of a compact camera are manufactured by a mixedmolten alloy of liquid phase and solid phase using the injection moldingmachine shown in FIG. 1. In these cases, good molding crystals can beobtained without filling defects even though the wall thickness is asthin as 0.7 mm.

Embodiment 8

[0101] Oxide films having 0.1 to 3 μm were formed on the surfaces of thevarious kinds of the products described in Embodiments 3 to 6 using theMg based alloys in accordance with the present invention by immersingthe products into aqueous solutions of 1M-Na₂MoO₄ and 1M-Na₂SO₄-0.5M.NaF(adjusting to pH 3.0 with H₂SO₄) at 60° C. for 180 seconds,respectively. The surface of the product is colored, and the thicknessof the film can be estimated from the tone of the color. The color ischanged from light brown to dark blown, and further to black dependingon the processing time. The obtained film showed good corrosionresistance, and had such an inert electric potential that the naturalimmersion electric potential 30 minutes after immersing into an aqueoussolution of 0.01M-Na₂B₄O₇ (pH 9.18) was higher than −1500 mV. Further,the oxide film was suitable as a based for coating with paint.

[0102] A water repellent fluoride film was further coated on the oxidefilm by being immersed into a solution dissolving perfluoro-hexane for24 hours and then by being heated at 150° C. for 10 minutes. The organicfilm had such a high water repellence that the contact angle with waterwas 120 to 130 degrees, and accordingly the durability could be furtherimproved.

[0103] According to the present invention, it is possible to obtain anMg based alloy which is low in melting point, good in fluidity atmolding, and good in mechanical property due to uniform and finestructure. Further, by reducing number of surface defects by improvingthe fluidity and by improving the dimensional accuracy by lowtemperature molding, the molding yield can be substantially improved.Furthermore, by reducing load to the metal members and the heatresistant members such as the mold and the cylinder of the injectionmolding machine, the lifetime of these members can be extended, andaccordingly the production efficiency of the magnesium based alloy partscan be improved.

[0104] In addition, according to the present invention, by forming theoxide film containing heavy metals having plural valences and enrichedwith Al in the based material on the surface of the Al containing Mgalloy through process in the solution, the oxide film can, serve as apaint based having good corrosion resistance. Further, the filmdescribed above can be fabricated without using any material harmful forthe environment.

[0105] By applying a general corrosion preventive paint or a waterrepellent paint onto the film, a better corrosion preventive coatingfilm can be obtained.

What is claimed:
 1. A method for using a high strength Mg based castingalloy which contains, by weight, more than 12%, and up to 17%, of Al;0.1 to 10% of Zn; 1 to 10%, of Sn; and 0.05 to 1.5% of Mn, said methodcomprising the step of injection molding the Mg based casting alloy. 2.A method for using a high strength Mg based casting alloy whichcontains, by weight, more than 12%, and up to 20%, of Al; 0.1 to 10% ofZn; 1 to 10%, of Sn; and 0.05 to 1.5% of Mn, and has crystal size of 10to 30 μm, said method comprising the step of injection molding the Mgbased casting alloy using a metal mold.
 3. A method for using a highstrength Mg based casting alloy which contains, by weight, 18 to 20% ofAl; 0.1 to 5% of Zn; 1 to 10%, of Sn; and less than 1.5% of Mn, and hasa tensile strength (x) at 20° C. larger than 240 MPa; and an elongation(y) larger than 0.5% and at the same time larger than a value calculatedby y=−0.295x+78, said method comprising the step of injection moldingthe Mg based casting alloy using a metal mold.
 4. A method for using ahigh strength Mg based casting alloy, which is injection molded using ametal mold, and which contains, by weight, 12 to 15% of Al; 0.1 to 5% ofZn; 1 to 10% of Sn; 0.1 to 0.5% of Mn, and the remainder contains Mgmore than 75%, said method comprising the step of injection molding theMg based casting alloy using a metal mold.
 5. A method for using a highstrength Mg based casting alloy which contains, by weight, 12 to 15% ofAl; 0.1 to 5% of Zn; 1 to 10% of Sn; 0.1 to 0.5% of Mn; at least oneelement selected from the group consisting of Ca, Si and rare-earthelements of which the total content is less than 5%; at least one kindof element selected from the group consisting of Sr and Sb of which thetotal content is less than 1%; and the remainder which is consistingessentially of Mg, said method comprising the step of injection moldingthe Mg based casting alloy using a metal mold.
 6. A method for using aMg based casting alloy, which contains, by weight, 12 to 20% of Al; and1 to 10%, of Sn, said method comprising the step of injection moldingthe Mg based casting alloy using a metal mold.
 7. A method for using aMg based casting alloy, which contains, by weight, 12 to 20% of Al; 1 to10%, of Sn; and less than 1.5% of Mn, said method comprising the step ofinjection molding the Mg based casting alloy using a metal mold.
 8. Amethod for using a high strength Mg based casting alloy, which contains,by weight, 12 to 15% of Al; 1 to 3% of Zn; 1.5 to 4.5% of Sn; 0.05 to0.5% of Mn; and the remainder which is consisting essentially of Mg,said method comprising the step of injection molding the Mg basedcasting alloy using a metal mold.
 9. A method for using high strength Mgbased alloy according to any one of claims 1 to 4, wherein the Mg basedcasting alloy contains one kind or more than two kinds of elementsselected from the group consisting of Ca, Si and rare-earth elements ofwhich the total content is less than 5% by weight; and at least one kindof element selected from the group consisting of Sr and Sb of which thetotal content is less than 1%.
 10. A method for using a Mg based castingalloy according to any one of claims 6 to 8, wherein the Mg basedcasting alloy contains one kind or more than two kinds of elementsselected from the group consisting of Ca, Si and rare-earth elements ofwhich the total content is less than 5% by weight; and at least one kindof element selected from the group consisting of Sr and Sb of which thetotal content is less than 1%.
 11. A die cast article produced by themethod for using a Mg-based casting alloy according to any one of claims1 to
 8. 12. A die cast article produced by the method for using aMg-based casting alloy according to claim
 9. 13. A die cast articleproduced by the method for using a Mg-based casting alloy according toclaim
 10. 14. The method for using a Mg-based casting alloy according toany one of claims 2, 6 and 7, wherein the alloy includes 12%-17% Al. 15.A method for using a Mg-based casting alloy according to any one ofclaims 1, 2, 6, and 7, wherein the Mg based casting alloy is moldedusing a semi-melted state where a solid phase and a liquid phase of analloy are mixed.
 16. A method for using a high strength Mg based castingalloy, which contains, by weight, more than 10%, and up to 17%, of Al;0.1 to 10% of Zn; 1 to 10%, of Sn; and 0.05 to 1.5% of Mn, whose surfaceis covered with an oxide film which contains Mg of 15 to 35% by atoms,said method comprising the step of injection molding the Mg basedcasting alloy using a metal mold.
 17. A method for using a high strengthMg based casting alloy according to claim 16, wherein said oxide filmfurther includes an oxide of Al of less than 15% by atoms.
 18. A methodfor using a high strength Mg based casting alloy which contains, byweight, more than 10%, and up to 17%, of Al; 0.1 to 10% of Zn; 1 to 10%,of Sn; and 0.05 to 1.5% of Mn, whose surface is covered with an inertoxide film having a natural immersion electric potential, 30 minutesafter immersing into an aqueous solution of 0.01 mol Na₂B₄O₇, pH of 9.2and a temperature of 25° C., which is greater than −1500 mV, said methodcomprising the step of injection molding the Mg based casting alloyusing a metal mold.
 19. A method for using a high strength Mg basedcasting alloy according to any one of claims 1 to 4, wherein the Mgbased casting alloy consists essentially of the Al, the Zn, the Sn, theMn and Mg.
 20. A method for using a high strength Mg based casting alloyaccording to claim 5, wherein the Mg based casting alloy consistsessentially of the Al, the Zn, the Sn, the Mn, the at least one elementselected from the group consisting of Ca, Si and rare-earth elements,and the at least one element selected from the group consisting of Srand Sb, and the Mg.
 21. A method for using a high strength Mg basedalloy, which contains, 12 to 20% of Al by weight, 0.1 to 10% of Zn byweight, 0.5 to 10% of Sn, and 0.05 to 1.5% of Mn; and the remainderwhich is consisting essentially of Mg, the method comprising the step ofinjection molding the Mg based casting alloy using a metal mold.
 22. Amethod for using a high strength Mg based casting alloy which contains,by weight, 12 to 15% of Al; 0.1 to 5% of Zn; 1 to 10% of Sn; 0.1 to 0.5%of Mn; at least one element selected from the group consisting of Ca, Siand rare-earth elements of which the total content is less than 5%; atleast one kind of element selected from the group consisting of Sr andSb of which the total content is less than 1%; and the remainder whichis consisting essentially of Mg, whose surface is covered with an oxidefilm which contains Mg of 15 to 35% by atoms, said method comprising thestep of injection molding the Mg based casting alloy using a metal mold.23. A method for using a high strength Mg based casting alloy whichcontains, by weight, 12 to 20% of Al; and 1 to 10%, of Sn, whose surfaceis covered with an oxide film which contains Mg of 15 to 35% by atoms,said method comprising the step of injection molding the Mg basedcasting alloy using a metal mold.
 24. A method for using a high strengthMg based casting alloy which contains, by weight, 2 to 20% of Al; 1 to10%, of Sn; and less than 1.5% of Mn, whose surface is covered with anoxide film which contains Mg of 15 to 35% by atoms, said methodcomprising the step of injection molding the Mg based casting alloyusing a metal mold.
 25. A method for using a high strength Mg basedcasting alloy which contains, by weight,12 to 15% of Al; 0.1 to 5% ofZn; 1 to 10% of Sn; 0.1 to 0.5% of Mn; at least one element selectedfrom the group consisting of Ca, Si and rare-earth elements of which thetotal content is less than 5%; at least one element selected from thegroup consisting of Sr and Sb of which the total content is less than1%; and the remainder which is consisting essentially of Mg, whosesurface is covered with an inert oxide film having a natural immersionelectric potential, 30 minutes after immersing into an aqueous solutionof 0.01 mol Na₂B₄O₇, pH of 9.2 and a temperature of 25° C., which isgreater than −1500 mV, said method comprising the step of injectionmolding the Mg based casting alloy using a metal mold.
 26. A method forusing a high strength Mg based casting alloy which contains, by weight,12 to 20% of Al; and 1 to 10%, of Sn, whose surface is covered with aninert oxide film having a natural immersion electric potential, 30minutes after immersing into an aqueous solution of 0.01 mol Na₂B₄O₇, pHof 9.2 and a temperature of 25° C., which is greater than −1500 mV, saidmethod comprising the step of injection molding the Mg based castingalloy using a metal mold.
 27. A method for using a high strength Mgbased casting alloy which contains, by weight, 2 to 20% of Al; 1 to 10%,of Sn; and less than 1.5% of Mn, whose surface is covered with an inertoxide film having a natural immersion electric potential, 30 minutesafter immersing into an aqueous solution of 0.01 mol Na₂B₄O₇, pH of 9.2and a temperature of 25° C., which is greater than −1500 mV, said methodcomprising the step of injection molding the Mg based casting alloyusing a metal mold.
 28. A method for using a high strength Mg basedcasting alloy according to any one of claims 1, 2, 4, 5, 6, 7 and 8,wherein said alloy has an elongation (y) larger than 0.5%.
 29. A methodfor using a high strength Mg based casting alloy according to any one ofclaims 1 to 8, wherein said alloy has an elongation (y) larger than3.5%.